A great deal of research has been conducted on lithium secondary batteries in which metallic lithium is used as the negative electrode because high voltage and high energy density are anticipated. A family of batteries, called variously lithium ion or rocking chair or swing systems, has been developed which include a carbon anode into which lithium intercalates. The ability to intercalate lithium is critical. Classically this requires a relatively good graphite structure. The intercalate is a true compound corresponding to a composition of C.sub.6 Li. It thus has safety and other advantages over a metallic lithium anode. However, the problem with batteries in which metallic lithium is used for the negative electrode is that cycle life is decreased because of reactions between the lithium and the solvent during charging and because of dendrite growth.
In order to eliminate this problem, studies have been conducted of Li-Al alloy and of various types of carbon fiber materials as negative electrode materials. However, new problems such as decrease in mechanical strength and deterioration of self-discharging characteristics have arisen with these materials.
As to the "rocking chair" cell, lithium is the only ionic species moving back and forth between the electrodes during charging and discharging. Specifically, during charging, intercalated lithium ions from the cathode e.g., LiMn.sub.2 O.sub.4, move through the electrolyte to the anode where they pick up an electron in the process of intercalating the carbon. During discharge the reverse reaction takes place, i.e., the uncharged lithium in the carbon loses an electron to the external circuit as it ionizes to Li+ which migrates to and enters the cathode concurrent with local reduction of the oxide lattice by an electron from the external circuit. The half cell reaction are shown below doing discharge.
LiC.sub.6 .fwdarw.Li+e.sup.- +C.sub.6 anode PA1 Li.sup.+ +Mn.sub.2 O.sub.4 +e.sup.- .fwdarw.LiMn.sub.2 O.sub.4 cathode
The prior art lithium ion system is characterized by an intercalatable carbon anode and an intercalatable variable valence metal oxide cathode usually also admixed with carbon for conductivity. It is the use of the carbon anode that sets the rocking chair system apart from earlier configurations using lithium metal anodes. These latter suffered from safety problems related to dendrite formation, flaking and spalling of lithium metal, leading, in turn to dangerously high reaction rates and to side reactions with the electrolyte. These safety problems have been largely sidestepped by the absence of a bulk lithium phase.
Development of new more oxidation resistant electrolytes and, longer range, the development of polymer electrolytes also contributes to safer Li cells. Nevertheless, these batteries utilize very reactive electrode materials and systems. Studies of even the Li intercalation electrode suggest it must be properly engineered to avoid runaway reaction under damage or short circuit conditions.
Although both electrodes of the lithium ion cell use carbon, these carbons, like most electrode materials, were selected from those commercially available. That neither anode nor cathode performance was satisfactory is seen from the poor power density and from the poor approach to theoretical energy density. The lithium ion battery as presently available has commercial utility only because of its inherent thermodynamics and rather in spite of less than optimal kinetics and engineering.
A target use for fibrils is electrodes and current collectors. Fibrils are ca. 100 .ANG. diameter, catalytically grown, graphitic fibers, typically several microns long. While fibrils are graphitic, geometric constraints force some differences with pure graphite. Like graphite, fibrils are composed of parallel layers of carbon but in the form of a series of concentric tubes disposed about the longitudinal axis of the fibers rather than as multi-layers of flat graphite sheets. Thus, because of the geometric constraints in the narrow diameter of the fibrils, the graphite layers cannot line up precisely with respect to the layers below as flat graphite sheets can. Convergent beam electron diffraction has confirmed that the (002) graphitic planes are oriented along the tube adds with high crystallinity.
The structure of fibrils compares quite closely with the much studied buckytubes. However, unlike buckytubes, fibrils are produced as indispersable aggregates uncontaminated with amorphous carbon allowing fibrils to be fashioned into electrode structures with only minimal processing. The fibrils are grown by contacting catalyst particles with gaseous hydrocarbon in a hydrogen rich atmosphere. Their diameters are determined by the size of the catalyst particles and average 7-12 nanometers. Lengths are several micrometers. They are hollow tubes with wall thicknesses 2 to 5 nanometers. The walls are essentially concentric tubes of individual graphite layers rolled into cylinders. At intervals along the length of a fiber some of the inner layers may curve into hemispherical cepta spanning the hollow interior. Near these, the walls may for a short distance change into nested cones. These reflect changes in the catalyst/carbon interface during growth of the fibril. Unlike other catalytic vapor grown carbon fibers they are free of less organized pyrolytic carbon on their surfaces.
Buckytubes, however, are grown by condensation of carbon vapor in an arc. They usually have a wider distribution of diameters from single layer walls to many tens of layers. The arrangement of graphite layers in the walls is very similar to fibrils. Some have only concentric cylinders (or polygonal cross sections). Others also have septa and nested cones. It is likely that some buckytubes are grown catalytically on catalyst particles derived from impurities in the source carbon or from the apparatus. It is not yet certain beyond doubt that any are self assembled without catalyst--spark temperatures are sufficient to vaporize iron or other particles after growth. Less organized carbon is deposited at the same time in the form of polygons or turbostratic carbon some of which may coat the buckytubes.
As would be expected from their structure and similarity to graphite, fibrils are conductive. While the conductivity of individual fibrils is difficult to measure, a recent attempt has yielded an estimated resistivity value of 9.5 (.+-.4.5) m.OMEGA.cm, a resistivity slightly higher than typically measured for graphitized carbon, but consistent with what has recently been recently measured for buckytubes.
Fibrils can be assembled into macrostructures composed of interconnected fibril nanotubes, similar to felt mats. The large number of contacts between individual, conductive fibrils in the fibril mat results in mats with high conductivities as well.
The porosity of homogeneous fibril mats is determined by overall mat density. Porosity can be further modified by co-slurrying fibrils with macrofibers (e.g., glass or carbon) before formation of the mat. This technique for forming both homogenous and co-slurried fibril mats is both convenient on the laboratory scale and is readily amenable to scale-up using fiber wet-laying (e.g., paper making).
Because of their small diameter, fibrils have a surface area of ca. 200 m.sup.2 /g as determined by BET measurement. The value for the surface area can also be arrived at by calculation based on average fibril dimensions. This agreement between calculation and the BET measurement demonstrates that the 200 m.sup.2 /g. is all on the external surface of the fibrils. Analytical electrochemistry of fibril mat electrodes demonstrate that all of the fibril surface area is available for electrochemical processes. For example, the double layer charging capacitance of fibril mat electrodes varies linearly with the mass of fibrils in the electrode over a wide range of fibril mat densities. Fibrils allow for a combination of constant pore size and high surface area that is not available in other conductive carbons.
Such open nets of fibrils impose their high external surface area and consequent electrochemical availability on any chemical system that can be deposited on or physically entangled within mats of them.
Currently available lithium ion batteries use an intercalatable carbon as the anode. The maximum energy density of such batteries corresponds to the intercalation compound C.sub.6 Li, with a specific capacity of 372 A-hours/kg.
The ability to intercalate lithium is critical. Classically, this requires a relatively good graphite structure. This intercalate is a true compound corresponding to a composition of C.sub.6 Li.